1. Field of the Invention
The present invention relates to an organic field effect element using an organic material.
2. Description of the Related Art
Conventional organic field effect elements using organic materials are described in, e.g., Published Unexamined Japanese Patent Application Nos. 61-202469, 63-14471, 63-14472, 1-89368, 1-259563, and 1-259564. These elements will be described below with reference to FIG. 1.
A gate electrode 2 is formed on a substrate 1, an insulating film 3 is formed to cover the entire surface of the gate electrode 2. As the insulating film, an oxide film or a polymer film such as a polysiloxane film is used. A source electrode 4 and a drain electrode 5 are formed separately to each other on the insulating thin film 3, and a semiconductor layer 6 consisting of a .pi.-conjugated polymer is formed to constitute a channel region between the source and drain electrodes 4, 5.
The operative principle of the organic field effect element in FIG. 1 will be described below. A current flowing in the channel is determined by a potential difference between the source and the drain electrodes 4, 5 and the resistance of the channel. In this case, the resistance of the channel is determined by the electric resistance of the .pi.-conjugated polymer when no voltage is applied to the gate electrode 2. On the other hand, when a voltage is applied to the gate electrode 2, carriers in the .pi.-conjugated polymer are attracted to and accumulated in the channel, thereby decreasing the resistance of the channel. In this manner, the current flowing in the channel can be controlled by the potential of the gate electrode 2 (e.g., A. Tsumura et al., Synthetic Metals, Vol. 25, pp. 11-23).
A field effect element is required to have the following characteristics. That is, a current (to be referred to as an ON current hereinafter) obtained when a voltage is applied to the gate electrode is large, and a ratio of the ON current to a current (to be referred to as an OFF current hereinafter) obtained when no voltage is applied to the gate electrode is large.
In a conventional organic field effect element, in order to increase an ON current, the carrier mobility in a channel must be increased, or the number of carriers accumulated in the channel must be increased. However, when the carrier mobility in the channel is increased, an OFF current is also increased. In addition, in order to increase the number of carriers accumulated in the channel without an increase in voltage applied to the gate electrode, the number of carriers in a .pi.-conjugated polymer must be increased. However, also in this case, an OFF current is increased. Therefore, in order to increase the ON current and to decrease the OFF current, a new means is required at present.
The above point is important when a field effect element is applied to particle use. As an example of an application of a field effect element, an active matrix-type drive circuit in a liquid crystal display is known. In this liquid crystal display, a field effect element is formed to each pixel, and a drain electrode and the pixel electrode are connected to each other, thereby controlling signal voltages applied to the pixel electrode by a gate electrode. The active matrix is better than a simple matrix. That is, the active matrix has a smaller leakage signal between pixels and higher image quality such as contrast, resolution, and pixel density than those of the simple matrix. In a conventional active matrix, a field effect element having an inorganic semiconductor thin film, such as amorphous silicon, has been used (e.g., Kaichi Fukuda et al., The Transactions of the Institute of Electrical Engineers of Japan, Vol. 110-A, No. 10, pp. 659-666 (1990)). Also in a field effect element used in the active matrix, in order to obtain a liquid crystal display having high quality such as a high contrast, a high resolution, and a high pixel density, an ON current must be increased, and an OFF current must be decreased, i.e., an ON/OFF current ratio must be increased. In addition, in order to increase a response of the liquid crystal display, the ON current must be increased (Masakiyo Matsumura et al., The Transactions of the Institute of Electrical Engineers of Japan, Vol. 110-A, No. 10, pp. 657-658 (1990)).
In recent years, there exists a demand for a liquid crystal display having a large area, and thus a glass or plastic substrate having a large area on which field effect elements are largely integrated has been required. However, it is very difficult to form an inorganic semiconductor such as an amorphous silicon into a thin film having a large area with uniform quality. In addition, since the substrate must be heated to 200.degree. C. or more in the process of manufacturing the inorganic semiconductor thin film, a plastic substrate cannot be used. Furthermore, the inorganic semiconductor thin film has a drawback that it is easily cracked by shock or expansion and contraction caused by heat resulting in poor reliability.
In order to solve the above problems, there is proposed an active matrix-type liquid crystal display using an organic field effect element having a .pi.-conjugated polymer thin film in place of an inorganic semiconductor thin film (e.g., Published Unexamined Japanese Patent Application Nos. 62-85224 and 1-259323). Since such an organic field effect element has advantages as compared with a field effect element using an inorganic semiconductor thin film, it is very important to increase an ON current to increase an ON/OFF current ratio. However, in the conventional organic field effect element, it is disadvantageously difficult to increase an ON current to increase an ON/OFF current ratio as above-described.